One convention does not an election make. Americans were treated to a lesson in the volatility of their own political responses the past two weeks. Surveys showed a substantial 12-point Reagan-Bush lead over Mondale-Ferraro going into the Democratic convention, a drawing close - even a slight 2-point Democratic edge - at the convention's close, and then a nearly total restoration of the Republican advantage a week later, to 10 points in the Gallup poll.
This describes a classic ''bubble'' effect as the public responded to the largely favorable and highly visible Democratic gathering in San Francisco. It was repeated in state surveys like the Connecticut Poll, where a pre-convention 20-plus Reagan-Bush edge shrank to 8 points the Monday after the convention, but widened again to 22 points by last Sunday.
What is important here are two things: The fundamentals of this election appear pretty well set. But with an electorate more loosely moored to party membership, there is a potential for wide swings, right through the election homestretch.
First the fundamentals, whose consistency is even more impressive than the evidence of short-term swings: Unless events intrude, the basic structure of the election reflects Mr. Reagan's advantage as incumbent, his personal popularity, the surge of positive national feeling during Reagan's tenure, and the long, steady economic recovery.
Reagan's personal popularity ratings have changed only a couple of points since last November, even through the Democratic convention bubble. The gap between those who think they will be better off a year from now and those who think they will be worse off has widened from nine points in the fall of 1981 to 40 points now in the optimists' favor. And so forth.
The Democrats acknowledged this consistency of the election's structure during their convention. They me-tooed the Republicans on defense, commitments abroad, social spending, patriotism, traditional values.
They sought to seize the initiative on the middle ground by challenging the incumbent on next year's Washington work - on deficits and taxes. While Republicans have given a number of me-too speeches in recent elections, Walter Mondale's was a novelty for Democrats. Both tickets are courting the same states - New Jersey, the big industrial states of Illinois and Ohio, and Florida and Texas in the South. They are both courting ethnic, Roman Catholic votes.
In other words, the pendulum swings in this election are working against the inertia of a powerful center.
The effect of Geraldine Ferraro's nomination has yet to become clear. The latest Gallup survey shows the gender gap all but disappearing, with Mr. Mondale gaining among men and Mr. Reagan gaining among women from pre-convention surveys. Other surveys show no such change. It may well be that the Ferraro choice will help Democratic candidates, generally, more than the Democratic presidential ticket, specifically.
Next it will be the Republicans' turn to spurt in the polls, with their national convention in Dallas later this month. There is no reason to think the stable factors in this election won't work on the Republicans, too, reducing any temporary gain.
In a struggle for the center ground in 1984, Reagan's incumbency and record give him an advantage. But with an electorate susceptible to short-term stimuli, his campaign aides are alert to the dangers of complacency.